1. Field of the Invention
This invention relates to an engine control system for a hybrid vehicle, and more particularly, to an engine control system which restarts the engine when the negative pressure or vacuum of the brake becomes insufficient when the engine is stopped, and during deceleration fuel supply cut.
This application is based on Japanese Patent Application No. 11-62411, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In the series hybrid vehicles, the engine drives a generator whose electric power output is used to drive the motor, which in turn drives the wheels. In the parallel hybrid vehicles, the motor coupled to the engine assists the rotation of the drive shaft of the engine while charging a battery with electric energy using the motor itself as a generator or using a separate generator.
Because the engines of such hybrid vehicles can constantly run within the engine-speed range of a high fuel mileage and a low emissions level or their drive loads can be reduced, the hybrid vehicles can achieve lower fuel consumption and lower emissions levels than conventional engine vehicles.
As disclosed in, for example, Japanese Unexamined Patent Application, First Publication Hei 8-317505, some of those hybrid vehicles are designed to be capable of stopping the operation of the engine in accordance with predetermined driving conditions.
Because of the ability to stop the operation of the engine, such a hybrid vehicle is superb in preventing over-charging of the battery or achieving a further improvement in fuel consumption but has the following problem.
As the throttle of a throttle-using engine, such as a gasoline engine, is manipulated in the closing direction, a negative pressure or vacuum is generally produced downstream of the throttle, so that such an engine is provided with a brake vacuum control unit which actuates a diaphragm by using the differential pressure between this negative pressure and the atmospheric pressure, thereby actuating the master cylinder in conjunction with the brake-pedal force. In a case where the engine of a hybrid vehicle equipped with the brake vacuum control unit stops operating under predetermined drive conditions, when the driver performs a brake manipulation, such as pumping, the negative pressure to be supplied to the master cylinder is reduced, making the absolute value of the negative pressure smaller. As a result, the required differential pressure between the negative pressure and the atmospheric pressure cannot be secured, lessening the capability to assist the brake-pedal force, which puts greater burden on the driver to manipulate the brake.
This kind of conventional technique is disclosed in, for example, Japanese Unexamined Patent Application, First Publication Sho 59-15650.
Accordingly, it is an object of the present invention to provide an engine control system for a hybrid vehicle, which can reduce the burden of brake manipulation on a driver.
To achieve this object, according to one aspect of this invention, an engine control system for a hybrid vehicle having an internal combustion engine (e.g., an engine E in one embodiment) and an electric motor (e.g., a motor M in the embodiment) as driving force sources for permitting stopping and starting of the engine in accordance with predetermined driving conditions, comprises: a brake booster (e.g., a brake vacuum control unit 4 in the embodiment) for receiving a negative pressure supplied by an operation of the engine; a pressure detector (e.g., a brake-vacuum sensor S6 in the embodiment) for detecting a pressure supplied to the brake booster; throttle-opening-state detector (e.g., a throttle-opening-state sensor S2 in the embodiment) for detecting a throttle opening state; and an engine-operation enable/disable determining device for determining whether or not to operate the engine when the engine is stopped, based on the throttle opening state (e.g., a throttle opening state TH in the embodiment) detected by the throttle-opening-state detector and the pressure (e.g., a brake-master-power negative pressure MPGA in the embodiment) detected by the pressure detector.
With the above structure, the engine-operation enable/disable determining device determines whether or not to operate the engine based on the pressure which is to be supplied to the brake booster and detected by the pressure detector and the throttle opening state detected by the throttle-opening-state detector.
The predetermined driving conditions for stopping the engine are, for example, based on the premise that the engine can be started by the motor (e.g., when a motor-start enable/disable flag F_MOTSTB is xe2x80x9c1xe2x80x9d in step S106 in the embodiment), the remaining battery charge (or the state of charge) SOC is equal to or above an over-discharge area (e.g., an energy-storage-zone flag F_ESZONE is xe2x80x9c0xe2x80x9d in step S107 in the embodiment), and an engine coolant temperature TW is equal to or higher than a predetermined level (e.g., the coolant temperature TW is equal to or higher than a coolant-temperature lower limit TWFCMG in step S108 in the embodiment).
The stopping of the engine operation includes conditions in which fuel supply is reduced or cut due to deceleration, from which the fuel supply returns to the normal state when a predetermined condition is met. When the conditions for stopping the engine are met (when an engine-stop control execution flag F_FCMG=1), fuel supply cut (fuel supply reduction) continues, and when continuous fuel supply cut makes it difficult to produce an engine load, resulting in a difficulty in supplying a negative pressure to the brake booster, the negative pressure can be generated by starting the engine. This helps to reduce the burden on the driver.
According to another aspect of this invention, the engine-operation enable/disable determining device: causes the engine to operate when the throttle opening state is other than completely closed (e.g., steps S203 and S213 in the embodiment), causes the engine to stop when the throttle opening state is completely closed and the pressure detected by the pressure detector is lower than a predetermined negative pressure (e.g., a brake-master-power negative pressure upper limit #MPFCMG at the time of stopping idling in the embodiment) which is equal to or lower than an atmospheric pressure (e.g., a decision in step S219 in the embodiment); and causes the engine to operate when the throttle opening state is completely closed and the pressure detected by the pressure detector is closer to the atmospheric pressure than the predetermined negative pressure which is equal to or lower than the atmospheric pressure (e.g., the decision in step S219 in the embodiment).
With this structure, when the driver performs a brake manipulation such as pumping so that the pressure detected by the pressure detector comes closer to the atmospheric pressure than the predetermined negative pressure, the engine is started. This can provide the negative pressure the brake booster calls for.
According to a further aspect of this invention, after the engine is restarted, the engine is not stopped until a vehicle speed exceeds a predetermined speed (a decision in step S119 in the embodiment).
This structure can prevent frequent repetition of the stop and start actions as in a case of heavy traffic, temporary stopping, restarting and so forth.